The invention relates to a colour display tube comprising a colour selection electrode having a frame comprising corner sections and diaphragm parts which are coupled to the corner sections.
The invention further relates to a colour selection electrode for use in such a colour display device and to a diaphragm part for use in said colour selection electrode.
A colour display tube as described in the opening paragraph is disclosed in U.S. Pat. No. 5,003,218. The colour display tube according to this specification is provided with a colour selection electrode having a frame consisting of four diaphragm parts and four corner sections, suspended in the corners of a display window.
The colour display tube described in U.S. Pat. No. 5,003,218 is provided with a colour selection electrode to ensure that electron beams coming from three electron guns, mounted in a neck portion of the tube, only excite electroluminescent material of one colour on the inner side of the display window. This colour selection is achieved by applying, for instance, a shadow mask in the tube. This mask comprises a large number of apertures, which are customarily arranged in either a slotted pattern or a dotted pattern. If the colour selection electrode is not stably positioned in the colour display tube, small deviations of its position will lead to a deterioration of the picture quality. When, for instance, the colour selection electrode is slightly deformed, the shadow effect of the colour selection electrode changes and consequentially, the electron beams do not hit the appropriate electroluminescent material on the display window. These registration errors cause discolouration of the display tube that leads to a deterioration of the quality of the picture on the colour display tube.
The colour display tube of the prior art has the disadvantage that the shadow mask, especially when this shadow mask is made from invarxe2x80x94which is a nickel-iron alloyxe2x80x94material, shows deformations during the manufacture of the colour display tube. In most high performance colour display tubes, the shadow mask as well the diaphragm parts are made of invar material, because the thermal expansion coefficient of invar is very low, resulting in a good doming performance. The corner sections of the colour selection electrode are normally made of iron because this offers an optimal stiffness of the colour selection electrode. After screen processing, i.e. the process wherein the black matrix and phosphor layers are applied to the inner side of the display window to form the screen, the display window is assembled with the funnel shaped part in a furnace. The high temperatures in this furnace cause permanent deformations in the frame of the colour selection electrode, due to the differences in expansion between the diaphragm parts and the corner sections.
It is an object of the invention to provide a colour display tube having an improved colour selection electrode with respect to the type described in the opening paragraph, which strongly reduces the registration errors on the display window.
According to the present invention, this object is achieved by means of a colour display tube which is characterized in that the diaphragm parts are provided with coupling means that increase the thermal stability of the frame.
The invention is based on the insight that the registration errors are diminished when the positional stability of the colour selection electrode with respect to the display window is increased during the heat treatment in the furnace where the display window and the funnel shaped part are assembled. This can be realized by providing the frame with coupling means for interconnecting the diaphragm parts and the corner sections in such a way that the differences in thermal expansion between the invar diaphragm parts and the iron corner section do not lead to a deformation of the frame of the colour selection electrode. In this way, the thermal stability of the frame is improved.
A preferred embodiment of the colour display tube according to the present invention is characterized in that the coupling means comprise an area for welding the diaphragm part to the corner section, which area further comprises slits.
When the area of the diaphragm part at which it is welded to the corner section is provided with slits, the diaphragm parts will not deform when the corner sections show a much larger expansion, because the slits in between different welding points ensure that the welds cannot shear.
In another embodiment, the coupling means comprise an area for welding the diaphragm part to the corner section, which area further comprises impressions.
This embodiment also prevents shearing of the welds between the diaphragm parts and the corner section. This can be attributed to the fact that the impression is stretched when the corner section expands more than the diaphragm part.
In a further embodiment, the diaphragm parts are welded to the corner sections by means of a plurality of spot welds.
The use of spot welds further increases the dimensional stability of the frame of the colour selection electrode, because the welds are small enough to prevent that stress occurs between the diaphragm part and the corner section at the location of the weld.
Yet another embodiment is characterized in that the diaphragm parts further comprise mathematical lines connecting the two spot welds of any pair, which mathematical lines are intersected by at least one slit or impression.
This embodiment has the advantage that in between each pair of welds a slit or impression is present. As a result, the differences in thermal expansion can be compensated for between all the welds. As a result, the diaphragm part will not be deformed and the positional stability of the frame is improved.
The invention also relates to the colour selection electrodes and the diaphragm parts used in a colour display tube according to the invention.
These and other aspects of the invention will be apparent from and elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter.